The use of pigmented or otherwise colored resin particles runs the gamut of literally thousands of different products in countless industries and uses. To mention but a few, we can point to pigmenting of synthetic resin fibers and films, and the coloring of plastic articles, literally by the tens of thousands.
The coloration of resin particles for use in the textile industry, in inks, various photographic applications, in the paper and pharmaceutical industry, in photocopying, magnetic tapes, medical diagnostic aids and tools is to mention but a few of the important uses of such particles.
Great effort, time, money and brain power have been expended to provide products whose requirements and characteristics are becoming more demanding and the uses more esoteric.
Since for many uses, phase stability of the final product is important if not critical (e.g. a paint pigment in a vehicle), in others uniformity of size and density, in still others the size per se, and, even further, the compatibility of the elements of the pigment particles, each on its own or any combination may and often is essential to optimization of the product for the intended use.
As discussed in U.S. Pat. No. 4,070,323 to Vanderhoff et al. (Jan. 24, 1978), the various techniques for producing polymer emulsions yield different particle sizes, thus:
1. direct emulsification of an organic solvent solution of the polymer in water using an oil-in-water type emulsifier to form droplets or particles of the polymer solution dispersed in water, after which the solvent may then be removed as by stripping or other method of removal. This method generally yields average polymer droplet sizes in the emulsion of about 1.mu. (micron) or larger.
2. inversion emulsification of the polymer solution by adding water thereto in the presence of an oil-in-water emulsifier which can function at least partially effectively as a water-in-oil emulsifier so that an emulsion of water-in-polymer solution is initially formed which, upon further addition of water, inverts to form a polymer solution-in-water emulsion. This method, however, generally calls for greater care and control than method (1) and in addition yields average polymer droplet sizes in the emulsion of about 0.8-1.0.mu. or larger.
3. emulsification by neutralization in which the polymer is prepared with functional acidic or basic groups and is emulsified in water by neutralizing these groups. Although this method can yield average polymer droplet sizes as small as 0.1.mu., films cast from such emulsions are usually water-sensitive due to the significant proportions of functional acidic or basic groups in the polymer.
It is accordingly generally preferable to employ the above direct emulsification method (1). The five-fold difference in particle size between latexes prepared by this method (minimum 1.mu.) and latexes prepared by emulsion polymerization (0.2.mu.) is however critical with respect to stability or resistance to settling or sedimentation. According to the Stoke's law, for spherical particles, EQU rate of sedimentation=(D.sup.2 /18.eta.)(d.sub.p -d.sub.m)g
where D is the particle diameter, .eta. the viscosity of the medium, d.sub.p and d.sub.m the densities of the particles and the medium, respectively, and g the gravitational constant.
The tendency for colloidal particles to settle upon standing is offset by their Brownian motion and the convection currents arising from small temperature gradients in the sample. The Brownian motion, which results from the unbalanced collisions of solvent molecules with the colloidal particles, increases in intensity with decreasing particle size. The convection currents depend upon the sample size and storage conditions. One criterion for settling is that a sedimentation rate of 1 mm. in 24 hours will be offset or nullified by the thermal convection currents and Brownian motion within the sample (Overbeek, in "Colloid Science, Vol. I", H. R. Kruyt, editor, Elsevier, Amsterdam, 1952, p. 80). Substituting this sedimentation rate in the above Stoke's equation enables determination of the largest particle size which, in any particular instance, will not settle out upon standing.
Thus, for polystyrene (density d.sub.p =1.05 gm/cm.sup.3) dispersed in water (density d.sub.m =1.00, viscosity n=1 cp), the largest particle size which will not settle on standing is 0.65.mu.. This calculated critical particle size is consistent with experimental observations that 1.0.mu. diameter monodisperse polystyrene latex particles settle out on standing within 1-3 months, 0.8.mu. diameter particles settle out within 3-6 months, and particles 0.5.mu. or smaller never settle out at all. As a matter of fact, 1.mu. diameter particles of most polymers, the minimum size generally produced by the direct emulsification method, settle at a relatively rapid rate which can be reduced by raising the viscosity of the water phase in some manner.
It is clear, therefore, that where optimum stability is required, it is necessary to utilize extremely small and uniform particles and where the particles are inorganic pigment and/or magnetic materials, and they are to be coated or encapsulated for use in the various areas described above, they must be extremely small, i.e. preferably below 0.5.mu., especially since they require a coating and usually a hydrophobic coating for their end uses, such coating, obviously, adding to the particle diameter.
Many have attempted to encapsulate pigment and/or magnetic particles in a hydrophobic polymer shell, with varying degrees of success. Among the reasons for this lies the fact that pigment and or magnetic particles have a hydrophilic surface which does not form a good basis for the formation of a hydrophobic polymer shell.
The incompatibility problem is but one of many. Major difficulties have also been encountered with the complexity of the encapsulation process, the non-uniform particle size distribution, the heterogeneous and uneven distribution of core materials inside the individual capsules, and the uneven shaped pigment-polymer particles which result.